Lithium-ion cells, since they have characteristics of high energy density, long life and the like, are used as power supplies for household appliances such as video cameras, and portable electronic devices such as laptop computers and cellular phones. Recently, the lithium-ion cells have been applied also to large-size cells mounted on electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like.
Lithium-ion cells are secondary cells having a following structure. In the charge time, lithium slips as ions out of a positive electrode and migrates to a negative electrode and is intercalated therein; On the other hand, in the discharge time, lithium ions reversely return from the negative electrode to the positive electrode, and their high energy density is known to be due to potentials of their positive electrode materials.
As positive electrode active substances of lithium-ion cells, there are known, in addition to lithium manganese oxide (LiMn2O4) having a spinel structure, lithium metal composite oxides having a layer structure, such as LiCoO2, LiNiO2 and LiMnO2. For example, LiCoO2 has a layer structure in which a lithium atom layer and a cobalt atom layer are alternately stacked through an oxygen atom layer. It is large in charge and discharge capacity and excellent in diffusability of lithium ion intercalation and deintercalation. For that reason, many of lithium-ion cells commercially available at present are lithium metal composite oxides having a layer structure, such as LiCoO2.
Lithium metal composite oxides having a layer structure, such as LiCoO2 and LiNiO2, are represented by the general formula: LiMeO2 (Me: transition metal). The crystal structure of these lithium metal composite oxides having a layer structure is assigned to a space group R-3m (“-” is usually attached on the upper part of “3,” indicating rotatory inversion. The same applies hereinafter); and their Li ions, Me ions and oxide ions occupy the 3a site, the 3b site and the 6c site, respectively. Then, these lithium metal composite oxides are known to assume a layer structure in which a layer (Li layer) composed of Li ions and a layer (Me layer) composed of Me ions are alternately stacked through an O layer composed of oxide ions.
As the lithium metal composite oxide having such a layer structure, although LiCoO2 is the mainstream at present, since Co is expensive, there has recently been paid attention to over-lithiated layered lithium metal composite oxides (referred to also as “OLO” or the like) in which Li is excessively added and the content of Co is reduced.
“xLi2MnO3-(1−x)LiMO2 solid solution (M: Co, Ni or the like)” known as an over-lithiated layered lithium metal composite oxide is a solid solution of a LiMO2 structure and a Li2MnO3 structure. The Li2MnO3 has a high capacity but is electrochemically inactive. By contrast, the LiMO2 is electrochemically active but has a low theoretical capacity. It is then reported that when an “xLi2MnO3-(1−x)LiMO2 solid solution (M: Co, Ni or the like)” is fabricated with the aim that by making the both into a solid solution, the electrochemically highly active property of the LiMO2 is utilized while the high capacity of the Li2MnO3 is brought out, a high capacity can thereby actually be obtained. It is specifically known that when the solid solution is charged at 4.5 V or higher, whereas the effective capacity of the LiCoO2 is 160 mAh/g, the effective capacity of the solid solution is improved up to about 200 to 300 mAh/g.
With respect to this kind of an over-lithiated layered lithium metal composite oxide, Patent Literature 1 discloses a positive electrode active substance composed of crystal particles of an oxide containing three transition metals and represented by Li[Lix(APBQCR)1−x]O2 (wherein A, B and C are each a different transition metal element; and −0.1≦x≦0.3, 0.2≦P≦0.4, 0.2≦Q≦0.4, and 0.2≦R≦0.4) wherein the crystal structure of the crystal particles is a layer structure, and the arrangement of oxygen atoms constituting the oxide is cubic closest packing. There is also disclosed a production method thereof in which when a oxide is coprecipitated, dissolved oxygen is removed from an aqueous solution by bubbling an inert gas such as nitrogen or argon in the aqueous solution, or a reducing agent is previously added to the aqueous solution and the oxide obtained by the coprecipitation and lithium hydroxide are dry mixed, heated at a stretch up to 1,000° C., calcined at the temperature for 10 hours, and when being cooled after the calcination is finished, once annealed at 700° C. for 5 hours, and then slowly cooled.
Further, Patent Literature 2 relates to a powder of a lithium metal composite oxide represented by LizNi1−wMwO2 (wherein M is at least one or more metal elements selected from the group consisting of Co, Al, Mg, Mn, Ti, Fe, Cu, Zn and Ga; and the followings are satisfied: 0<w≦0.25, and 1.0≦z≦1.1), and discloses a positive electrode active substance for a nonaqueous electrolyte secondary battery wherein the powder is constituted of secondary particles formed by aggregation of a plurality of the primary particles of the powder of the lithium metal composite oxide; the shape of the secondary particles is spherical or ellipsoidal; 95% or more of the secondary particles have a particle diameter of 20 μm or smaller, and the average particle diameter of the secondary particles is 7 to 13 μm; the tap density of the powder is 2.2 g/cm3 or higher; the average volume of pores having an average diameter of 40 nm or smaller in a pore distribution measurement using a nitrogen adsorption method is 0.001 to 0.008 cm3/g; and the average crushing strength of the secondary particles is 15 to 100 MPa. A production method of the positive electrode active substance for a nonaqueous electrolyte secondary battery is also disclosed, the method comprising: a step 1 of fabricating a metal composite hydroxide containing Ni and metal M (wherein M is at least one or more metal elements selected from the group consisting of Co, Al, Mg, Mn, Ti, Fe, Cu, Zn and Ga) and having a tap density of 1.7 g/cm3 or higher; a step 2 of weighing and mixing the metal composite hydroxide obtained in the step 1 and lithium hydroxide so that the ratio of the number of Li atoms to the total number of Ni atoms and metal M atoms becomes 1.0 to 1.1 to thereby obtain a mixture; and a step 3 of heating the mixture at a temperature-rise rate of 0.5 to 15° C./min from room temperature up to 450 to 550° C. and holding the mixture at the reached temperature for 1 to 10 hours to thereby carry out a first-stage calcination, thereafter further heating the resultant at a temperature-rise rate of 1 to 5° C./min up to 650 to 800° C., holding the resultant at the reached temperature for 0.6 to 30 hours to thereby carry out a second-stage calcination, and thereafter furnace-cooling the resultant to thereby obtain the positive electrode active substance for a nonaqueous electrolyte secondary battery.
Patent Literature 3 discloses a lithium metal composite oxide represented by the formula Li1+xNiαMnβCoγO2 (wherein x is in the range of about 0.05 to about 0.25; α is in the range of about 0.1 to about 0.4; β is in the range of about 0.4 and about 0.65; and γ is in the range of about 0.05 to about 0.3). A production method thereof is disclosed in which a metal salt of desired molar ratio is dissolved in an aqueous solvent such as purified water; then, the pH of the solution is regulated by adding Na2CO3 and/or ammonium hydroxide to thereby precipitate a metal carbonate salt having a desired amount of the metal element; the precipitated metal carbonate salt is separated from the solution, cleaned and dried to thereby form a powder; after the drying, the recovered metal carbonate salt powder and a Li raw material are mixed, subjected to a heat treatment at about 400° C. to 800° C., and further calcined at a temperature of about 700° C. to 1,200° C. to thereby obtain the lithium metal composite oxide.